1. Field of the Invention
The present invention relates to improvements in conductive vias of ceramic packaging substrates. More particularly, the present invention relates to improvements in multilayer ceramic packaging substrates or bodies having vertical conductive vias which provide surface electrical interconnection with one or more internal printed circuits insulated between dielectric layers of the substrate or body. Multilayered circuitized ceramic substrates contain via-connected patterned metal layers which act as electrical conductors sandwiched between ceramic layers which act as electrical insulators. These ceramic substrates are formed by providing thin green (meaning unfired) dielectric ceramic sheets of glass particles or an admixture of glass and crystalline particles mixed With binders, with vertical via through-holes, filling such holes with conductive via-fill composition, and printing surfaces of such dielectric sheets with patterns or circuits of conductive ink containing metal particles for forming conductors between the ceramic greensheets. Said patterned and personalized greensheets are aligned, stacked in proper sequence, and then laminated with heat and pressure to form multilayer laminates. This green laminate is fired to burn off the binder materials, and to sinter the ceramic and metallic particulates into dense conductive wiring and insulating dielectric. Terms such as ceramic and glass-ceramic are often used interchangeably in the art. To avoid confusion for the purpose of this application the following definitions will be used. The term ceramic means an aggregate of randomly oriented crystalline particles, with interstices that may contain uncrystallized material such as glass or pores of void space. The terms coalescence or densification refer to a heat treatment to reduce the number and size of the pores in greensheets during processing. The term crystallization refers to the conversion of glass to crystallites upon further heating, preferably after coalescence or densification. The term sintering refers to the heat treatment required to densify the final ceramic. The term sintering temperature means, for a laminate not requiring crystallization, the coalescence temperature. Substrates made of ceramics requiring very high temperatures for particle coalescence and densification, such as alumina, restrict the choice of cosinterable conducting metallurgies to high melting point metals, for example refractory metals, such as molybdenum, tungsten, platinum, palladium or a combination of these with each other or certain other metals and preclude the use of preferable electrical conductors such as gold, silver and copper which have melting points less than the alumina sintering temperature. Alumina is a good insulator, has high thermal conductivity and has good strength. The dielectric constant of alumina is high, about 10, and results in slow electric signal propagation in circuits supported by the alumina.
Materials often referred to as glass-ceramics have been intensively studied in recent years for use as circuit substrates. These ceramics generally have a low dielectric constant, a low thermal coefficient of expansion which is close in value to silicon. and a low sintering temperature. The low sintering temperature permits the use of low melting point metals, such as copper and certain noble metals, for electrical conductors. The noble metals gold and silver have low resistivities comparable to that of copper. However, copper is less expensive and, therefore, its use substantially reduces manufacturing cost. When an oxidizable metal, such as copper, silver, molybdenum, etc., is used as the electrical conductor, it is necessary that thermoplastic organic binder materials contained within the green sheet used to form the ceramic and contained within the paste used to form the conductors be depolymerized and burned out in an atmosphere and at a temperature wherein the metal is not substantially oxidized.
Failure to burn out binders results in less than desirable ceramic properties. For example, if the binder is not fully burned out, residual carbon left in the sintered ceramic modifies the ceramic dielectric constant and inhibits complete densification. With as little as 0.1% residual carbon content the ceramic may be black, having an apparent dielectric constant greater than 1000 and, rather than being an insulator, the fired ceramic will be conductive and lossy. An at least partially oxidizing atmosphere is generally needed to burn out the binder.
Removal of the binder is complicated by the additional requirement that the burn-out ambient not excessively oxidize the oxidizable metal lines and planes. If the metal is excessively oxidized, the metal oxide can diffuse into the ceramic and change the dielectric properties of the ceramic. Also, when metal is excessively oxidized it expands causing stress within the laminate which can result in cracking of the laminate in the area of the via holes.
An additional complication is cracking, which can initiate on cooling due to the significant difference in thermal expansion coefficients (TCE) for conductors, such as copper, and glass-ceramics. In earlier molybdenum/alumina multilayer ceramics, this problem was obviated because the TCE values of the conductor and ceramic were similar. The contraction of the metal-bearing paste in the via areas causes separation from the dielectric layers and loss of the hermetic seal which is necessary to prevent porosity and leakage that allow the intrusion of post-sintering process chemicals and ambient moisture.
2. Description of the Prior Art
Reference is made to U.S. Pat. Nos. 4,547,625 and 4,598,167 for their disclosures and illustration of multilayer ceramic packaging substrates or bodies having a plurality of surface-exposed vertical conductive vias to provide interconnection between conductive areas of surface-mounted chips and different electrically-conductive line patterns or circuits printed between different dielectric layers of the structure. The ceramic compositions of these patents are low temperature softening borosilicate glasses, and the via compositions comprise a major percent by weight of conductive metal powder, i.e., between about 70% and 95% by weight copper, and a minor percent by weight of the low temperature softening glass, in order to provide bodies in which the difference between the coefficient of thermal expansion of the via composition and that of the ceramic layer composition is less than 1OO.times.1O.sup.-7 /.degree. C. The objective is to produce bodies which do not crack in the via areas when the bodies are fired and thermally-cycled.
U.S. Pat. Nos. 4,098,949; 4,301,324 and 4,594,181 also disclose multilayer circuit bodies containing conductive compositions. The compositions of 4,098,949 are pastes containing 50-88.degree. % by weight of metal powder, 2-40 % by weight of ground glass frit of silicon dioxide and other oxides, and 5-48% by weight of liquid vehicle. The glass frit is a mixture of particles up to 325-mesh (45.mu.m) size and the metal powder is 200-mesh (75.mu.m) size or smaller. The conductive compositions of U.S. Pat. No. 4,301,324 are pastes of conductive metal powders which are extruded into the via holes of glass/ceramic layers which contain crystalline phases and which produce glass/ceramic bodies having a very stable thermal expansion coefficient over a wide sintering temperature range. The addition of suitable glass frits to the metal powder paste is suggested to enhance the bonding between the metal and the glass/ceramic of the dielectric circuit body.
The conductive compositions of U.S. Pat. No. 4,594,181 comprise conductive metal particles coated with certain oxides in order to modify their sintering and shrinkage properties to more closely match those of the green ceramic composition used to form the dielectric layers of the body. Copper particles or spheres having an average particle size between 0.5 and 20.mu.m are disclosed. A process for sintering a metal member bonded to a substrate of U.S. Pat. No. 4,599,277 utilizes an organometallic compound to cause the densification temperature of the metal to be closer to that of the substrate, thereby obviating unwanted differential shrinkage rates. The ceramic/glass dielectric compositions of U.S. Pat. No. 4,301,324, discussed above, and the processes for producing multilayer interconnected thick film circuit patterns therefrom are suitable for use according to the present invention, and therefore the disclosure of U.S. Pat. No. 4,301,324 is hereby incorporated herein by reference thereto. The present invention represents an improvement over said patent with respect to the substitution of the novel conductive metal paste compositions in place of those disclosed by said Patent.